Pulse-width modulation (PWM) signals are commonly used in a variety of electrical applications, such as signal processing channels and motor speed control circuitry. A PWM signal alternatively transitions between two voltage levels ("high" and "low" levels) over each of a series of successive cycles, with the cycles occurring at a selected frequency. The percentage of time that the signal is at the high level as compared to the duration of the entire cycle is commonly referred to as the duty cycle. Thus, a PWM signal with a 50% duty cycle resembles a typical square-wave signal with high and low levels of equal length.
One particularly useful application of PWM signals is in the area of motor driver circuitry for hard disc drive data storage devices ("disc drives"). A typical disc drive includes one or more rigid discs which are mechanically coupled for rotation about a spindle motor at a selected operational speed (such as 10,000 revolutions per minute). Data are magnetically stored and retrieved from tracks defined on the disc surfaces by an array of read/write heads which are controllably positionable using an actuator motor, such as a voice coil motor (VCM).
For each type of motor, it is common to generate a multi-bit digital control value indicative of the desired speed of the motor. The disc drive converts the digital control value to a PWM signal with a duty cycle indicative of the magnitude of the digital control value. In turn, the PWM signal is converted to an analog control voltage with a magnitude indicative of the PWM duty cycle, with the analog control voltage used to control the application of current to the motor. It will be recognized that in the case of the spindle motor, the analog control voltage will tend to made necessary adjustments in the rotational speed of the spindle motor to maintain this speed within a desired range, whereas in the case of the VCM, the analog control voltage controls the acceleration and deceleration of the heads as the heads are moved to positions adjacent various tracks on the discs.
Prior art circuits used to convert a PWM signal to a corresponding analog voltage have taken a variety of forms, including use of filtering to achieve the necessary conversion. While operative, limitations with such prior art methodologies include insufficient bandwidth and resolution, as well as difficulties in efficiently implementing the entire circuit into an application specific integrated circuit (ASIC). Accordingly, there is a need for an improved approach to converting a PWM signal to a corresponding analog voltage, and it is such improvements that the present invention is directed.